[music-dsp] IIR allpass

[Ted David]

"James Chandler Jr." wrote:

> The Q setting controls how fast the phase "turns around" in the vicinity of
> the filter's Fc (center frequency). Higher Q causes more of the phase shift
> to occur in the vicinity of Fc.

Why is Q of interest in context of an all-pass?  My definition of an all-pass
includes a statement that it has a flat frequency response.  My definition of
Q includes a statement about the connection between the ratio of a filter's
center frequency and its  -3dB response bandwidth.  That would seem to
indicate that an all-pass has a fixed Q.  For example, an ideal brick wall
digital filter would have a Q of 0.5, due to Nyquist.  In a truly infinite
bandwidth all-pass it would be zero.  Please elaborate on why Q is of
interest for an all-pass.

Just trying to get a little more educated

Ted David